Synthetic fiber felts are useful as filters and are well-known in the art. U.S. Pat. No. 4,361,619 to Forsten, et al. discloses a filter of poly(tetrafluoroethylene) and glass fibers made by blending 1-35 weight percent glass fibers and 65-99 weight percent poly(tetrafluoroethylene) fibers to make a carded web which was crosslapped to form a batt and then needled to form a felt. This crosslapped batt could also be needled to a supporting scrim of woven poly(tetrafluoroethylene) to form a felt or felted scrim
U.S. Pat. No. 4,361,619 further disclosed in Example 2 the carding of blends containing straight glass fiber and crimped PTFE fiber. The patent states that even with the addition of a crimped fiber blends of more than 50 weight percent glass fiber were extremely difficult to card. Experience of those skilled in this art has shown that the term "extremely difficult" in this context means that the blend may not process through the card, that the quality of the carded web may not be sufficient to make a useful product, or that the speed of the card must be significantly reduced in order that any arrangement of fibers resembling a web be produced. The maximum speed of the card that can be achieved when the concentration of glass fibers is greater than 50% by weight of the blend is in the range of 1 meter/minute. The overall result has been that carding blends that contain more than 50% by weight glass fiber has not been practical.
The difficulty in making felts containing more than 50% by weight glass fiber has been attributed to the lack of crimp in glass fiber. Accordingly, those skilled in the art related to making glass fibers have suggested that curly glass fibers would be more processible in forming felts than straight fibers. This has not proven to be the case.
Curly or irregularly shaped fibers were made by spinning a combination of two glasses into a fiber. U.S. Pat. No. 2,998,620 to Stalego discloses curly glass fibers of dual glass compositions. Stagelo teaches producing curly fibers by passing two glass compositions, each having different coefficients of thermal expansion, through the orifices of a spinner. The glasses are extruded in aligned integral relationship such that these fibers curl naturally upon cooling.
U.S. Pat. No. 3,073,005 to Tiede discloses an improved, faster process for making bicomponent (dual) curly glass fiber compositions of Stalego
Both Stalego and Tiede disclose an improvement in the carding of these bicomponent glass fibers when crimped soluble fibers are blended with the glass fibers.
Additional work has been performed by others, and most recently International Patent Applications WO95/12554, which discloses glass compositions which make useful dual glass fibers, and WO95/29880, which discloses dual glass fibers having special bonding properties have been published.
The primary producer of curly glass fibers, Owenings-Corning Fiberglas Corporation of Toledo, Ohio, has disclosed in published articles (such as Mechanical Engineering, volume 117, number 2, page 38, February 1995) that their dual glass fiber, sold under the trademark MIRAFLEX, has high resilience and is recommended for use in insulation. In addition it has been stated that this dual glass fiber can be carded and needled in traditional textile processes. However, attempts, by the present inventor and others, to actually card such dual glass fibers as MIRAFLEX at commercially accepted carding speeds have not heretofore been successful. The high resilience of the curly glass fiber acts to preclude normal carding. Art applicable to low modulus fibers such as British Patent No.1,030,570, which teaches an improved carding process for nylon, is not applicable to the very high modulus, fragile glass fiber.